1. Field of the Invention
The present invention relates to a device for applying at least one microdroplet to a substrate and it relates to a dosing head which is adapted to be used in such a device. The present invention relates additionally to a method of applying at least one microdroplet to a substrate. In particular, the present invention relates to devices and methods which are suitable for producing so-called biochips in the case of which a plurality of different analytes are applied to a substrate so as to detect different substances in an unknown sample.
2. Description of Prior Art
The increasing degree to which the genomes of human beings, animals and plants are deciphered creates a large number of new possibilities from the diagnosis of genetically based illnesses to the much faster search for pharmaceutically interesting agents. For example, the above-mentioned biochips will be used in the future for examining food with regard to a large number of possible, genetically modified constituents. In another field of use, such biochips can be used for detecting the precise genetic defect in the case of genetically based illnesses so as to derive therefrom the ideal strategy for treating the illness.
The biochips which are adapted to be used for these applications normally consist of a carrier material, i.e. a substrate, having applied thereto a large number of different substances in the form of a raster. Typical raster distances in the array range from 100 xcexcm to 1,000 xcexcm. The variety of different substances, which are referred to as so-called analytes, on a biochip ranges from a few different substances to a few 100,000 different substances per substrate, depending on the respective case of use. Each of these different analytes can be used for reaffirming the presence of a specific substance in an unknown sample.
When an unknown sample liquid is applied to a biochip, reactions occur in the case of specific analytes; these reactions can be detected through suitable methods, e.g. fluorescence detection. The number of different analytes on the biochip corresponds to the number of different constituents in the unknown sample liquid which can be analyzed simultaneously by means of the respective biochip. Such a biochip is therefore a diagnostic tool with the aid of which an unknown sample can be examined with regard to a large number of constituents simultaneously and purposefully.
For applying the analytes to a substrate for the production of such a biochip, three fundamentally different methods are known at present. These methods are used alternatively, depending on the number of biochips required and depending on the necessary number of analytes per chip.
The first method is referred to as xe2x80x9ccontact printingxe2x80x9d; this method uses a bundle of steel capillaries filled with different analytes in the interior thereof. This bundle of steel capillaries is stamped onto the substrate. When the bundle is being removed, the analytes adhere to the substrate in the form of microdroplets. In the case of this method, the quality of the printing pattern is, however, influenced very strongly by the effect of capillary forces and, consequently, it depends on a large number of critical parameters, e.g. the quality of and the coating on the surface of the substrate, the precise geometry of the nozzle and, primarily, the media used. In addition, the method is very susceptible to contaminations of the substrate and of the nozzles. The number of analytes which can be dealt with by the above-described method is up to a few hundred per substrate.
In a second method of producing biochips, which is normally referred to as xe2x80x9cspottingxe2x80x9d, so-called microdispensers are used in most cases, which, similar to ink-jet printers, are capable of firing individual microdroplets of a liquid onto a substrate in response to a suitable control command. Such a method is called xe2x80x9cdrop-on-demandxe2x80x9d. Such microdispensers are commercially available from some firms. The advantage of this method is to be seen in the fact that it permits a contact-free application of the analytes to a substrate, the influence of capillary forces being then of no importance. An essential problem is, however, that it is very expensive and extremely difficult to arrange a large number of nozzles, which have each supplied thereto a different medium, in parallel and in an array, respectively. The limiting element is in this case the actorics and the media logistics, which cannot be miniaturized to the desired extent.
The so-called xe2x80x9csynthesis methodxe2x80x9d is nowadays used as a third method of producing biochips; in this method, the analytes, which normally consist of a chain of linked nucleic acids, are produced chemically on the substrate, i.e. they are synthesized. For delimiting the spatial position of the different analytes, methods are used which are known e.g. from the field of microelectronics, e.g. lithographic methods with masking techniques. Among the methods mentioned hereinbefore, this synthesis method is by far the most expensive one, but it permits the production of the greatest variety of analytes on a chip, the order of magnitude of this variety being 100,000 different analytes per substrate.
It is the object of the present invention to provide methods and devices by means of which microdroplets can be applied individually or in the form of a regular pattern to a substrate inexpensively and accurately.
In accordance with a first aspect of the present invention, this object is achieved by a device for applying at least one microdroplet to a substrate, comprising:
a dosing head with at least one nozzle orifice; and
a drive means for applying to the dosing head an acceleration of such a nature that a microdroplet is driven out of the nozzle orifice and onto the substrate due to inertia.
In accordance with a second aspect of the present invention, this object is achieved by a dosing head comprising a dosing head body in which the at least one nozzle orifice is formed, said dosing head body including in addition a liquid storage area which is in fluid communication with the nozzle orifice in such a way that, due to the inertia of a liquid present in said liquid storage area, a microdroplet can be driven out of said nozzle orifice by applying to the dosing head an acceleration.
The liquid storage area of the inventive dosing head can preferably be formed by a standpipe extending away from the nozzle orifice in a direction opposite to the direction in which the microdroplet can be driven out of the dosing head.
In accordance with a third aspect of the present invention, this object is achieved by a method of applying at least one microdroplet to a substrate, said method comprising the following steps:
a) filling a liquid storage area, which is in fluid communication with a nozzle orifice, with an amount of liquid, said nozzle orifice and said liquid storage area being formed in a dosing head; and
b) applying to said dosing head an acceleration of such a nature that a microdroplet is driven out of the nozzle orifice due to the inertia of the amount of liquid.
Hence, the present invention provides devices and methods by means of which biochips can be produced at a reasonable price and in high numbers of pieces. The present invention is based on the finding that microdroplets can be driven out of the dosing head by means of a mechanical acceleration which is applied to a dosing head by an external mechanical system. Suitable devices of an arbitrary nature can be used for the external mechanical system, which represents a drive means, these suitable devices being e.g. piezo-bending transducers, piezo-stacks, pneumatic drives and the like. A liquid contained in areas which are in fluid communication with the nozzle orifice is then acted upon by inertia forces, these areas being e.g. the nozzle itself, a media line and a reservoir. Since the liquid is not rigidly connected to the dosing head, these inertia forces have the effect that the liquid is accelerated relative to the dosing head carrying the liquid. Hence, the liquid starts to move relative to the dosing head. When this relative movement between the liquid contained in the nozzle and the nozzle orifice is sufficiently large, a microdroplet will tear off at the nozzle. The size of this drop is determined by the magnitude and the duration of the accleration of the dosing head, the size of the liquid mass whose inertia causes the ejection, the diameter of the nozzle and the flow resistance of the movement of the liquid in the dosing head. The direction of the acceleration applied to the dosing head must be oriented such that the liquid is flung out of the nozzle due to its inertia but does not withdraw into liquid storage areas or media lines in the dosing head.
By means of the present invention, a plurality of microdroplets is preferably applied to a substrate simultaneously so that e.g. a biochip for which different biologically relevant substances are applied to a substrate in a regular pattern can be produced inexpensively and reliably. Due to the acceleration of the dosing head, it is possible to drive a microdroplet out of each individual nozzle in a dosing head at the same time, the inertia of the liquid being utilized for this purpose.
The dosing head can have applied thereto different accelerations so as to cause an ejection of liquid droplets. One possibility is to accelerate the dosing head from a position of rest adjacent the substrate very strongly so as to cause the dosing head to move away from the substrate. An alternative possibility is to decelerate the dosing head abruptly while it is carrying out a continuous movement towards the substrate; this deceleration can be supported e.g. by a mechanical stop. In addition, it is also possible to provide a mechanically sufficiently stiff holder for the dosing head, which is excited in ranges of the natural frequency thereof in such a way that the holder and, consequently, the dosing head carry out half an oscillation. In this case, the maximum accleration occurs at the point of reversal of the oscillation so that the holder and the dosing head are arranged such that the dosing head is disposed adjacent the substrate at the point of reversal of the oscillation.
The dosing head according to the present invention may comprise one or a plurality of nozzles, the nozzles being arranged at a distance from one another which corresponds to the distance at which the fluid droplets to be applied to the substrate are to be spaced from one another on this substrate. When the nozzles are to be arranged in very closely spaced relationship with one another, i.e. when a very narrow array of different substances is to be produced on the substrate, it will be advantageous when each nozzle is connected to a larger external reservoir via a media line of its own; through these external reservoirs, each nozzle can be supplied with a specific liquid. When the distances between the individual nozzles are large enough for permitting the nozzles to be supplied with liquid by conventional methods, e.g. by standard automatic pipetting devices, the media lines and the external liquid reservoirs can be dispensed with. In this case, the liquid reservoir can be arranged directly above the nozzle.
Such a dosing head can now be decelerated abruptly directly in front of the substrate while carrying out a movement towards the substrate. Due to its inertia and in view of the fact that it is not rigidly connected to the dosing head, the liquid continues this movement and is flung out of the nozzle and onto the substrate. Alternatively, a static dosing head, which is located directly above a substrate, can be accelerated abruptly in a direction away from the substrate. Due to its inertia and in view of the fact that it is not rigidly connected to the dosing head, the liquid cannot follow this movement. It leaves the nozzle in a direction opposite to the direction of movement of the dosing head and is first suspended freely in space before the drops fall onto the substrate due to the force of gravity, the movement of the dosing head being caused by the acceleration away from the substrate. A device can be provided for producing an electrostatic field between the dosing head and the substrate so as to support the application of the droplets to the substrate in this way.
In each of the two above-mentioned cases, it will be advantageous when the acceleration of the dosing head is effected at a position at which the distance between the nozzles in the dosing head and the substrate is very small. This guarantees that, if satellite drops form during the separation of the microdroplets, these satellite drops will merge with the mother drop on the substrate at the latest. Due to the small distance, it is guaranteed that the satellite drops will land on the mother drop even if they left the nozzle at a slightly different angle.
It follows that the present invention provides devices and methods by means of which microdroplets, especially microdroplets of biologically relevant substances, can be produced and applied to a substrate in a regular pattern.